1. Field of the Invention
The present invention relates to a complementary metal-oxide semiconductor (CMOS) image sensor and a method for fabricating the same, and more particularly, to a CMOS image sensor and a method for fabricating the same in which lifting of the microlenses and diffused reflection of microlenses are prevented.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device that converts optical images to electrical signals. The image sensor is classified into a charge coupled device (CCD) and a CMOS image sensor. The CCD stores charge carriers in MOS capacitors and transfers the charge carriers to the MOS capacitors. The MOS capacitors are proximate to one another. The CMOS image sensor employs a switching mode that sequentially detects outputs of unit pixels using MOS transistors by forming the MOS transistors to correspond to the number of the unit pixels using CMOS technology, which uses a control circuit and a signal processing circuit as peripheral circuits.
The CMOS image sensor that converts data of an object into electrical signals includes signal processing chips having photodiodes. Each of the signal processing chips includes an amplifier, an analog-to-digital converter (A/D converter), an internal voltage generator, a timing generator, and a digital logic. The CMOS image sensor is thus economical in view of space, power consumption, and cost. The manufacture of the CCD requires technical and complicated processing steps. However, the CMOS image sensor can be manufactured in mass production by a simple etching process of a silicon wafer cheaper than that of the CCD. Also, the CMOS image sensor has an advantage in its packing density. Thus, the CMOS image sensor is widely used for various application fields such as digital still cameras, smart phones, PDAs, notebook computers, surveillance cameras, bar-code detectors, and toys.
To enhance light sensitivity of the CMOS image sensor, there are efforts to increase a fill factor of an area occupied by a photodiode among the whole area of the image sensor. However, such efforts have limitations because a limited area is needed for a logic circuit for signal processing. Therefore, it is necessary to change a path of incident light to an area other than the photodiode so as to condense the incident light onto the photodiode. To condense the incident light onto the photodiode, a microlens is generally used. The microlens is formed on a nitride layer used as a passivation layer. The nitride layer has poor adhesion to a photoresist, which is used as a main component of the microlens. For this reason, lifting of the microlens is caused. Also, if a metal line of the image sensor is formed in the light path, diffused reflection occurs and deteriorates the quality of the image sensor.
A related art CMOS image sensor will be described with reference to the accompanying drawings.
FIG. 1 is a sectional view illustrating a related art CMOS image sensor.
As shown in FIG. 1, a red photodiode 11 is formed on a semiconductor substrate 10 in which a first epitaxial layer (not shown) is grown. A second epitaxial layer 12 is grown on the first epitaxial layer including the red photodiode 11. A green photodiode 13 is formed in the second epitaxial layer 12, and a third epitaxial layer 14 is grown on the second epitaxial layer 12 including the green photodiode 13. A blue photodiode 15 and a trench are respectively formed in the third epitaxial layer 14. The trench serves to insulate fields from each other. Shallow trench isolations (STI) regions 16 filled with an insulating material are formed.
An interlayer insulating layer 17 is deposited on the third epitaxial layer 14. A first metal layer (not shown) is formed on the interlayer insulating layer 17 and then is patterned to form metal lines 23. The process steps of forming the interlayer insulating layer 17 and the metal lines 23 are repeated several times to deposit a desired number of the metal lines 23. A second metal layer (not shown) is formed on the deposited interlayer insulating layer 17 and then is patterned to form a metal pad 24. A first insulating layer 19 may be formed of an oxide layer on the deposited interlayer insulating layer 17, and a second insulating layer 20 is formed on the first insulating layer 19. The second insulating layer 20 serves as a passivation layer that protects the image sensor from water or physical impact. Subsequently, the first insulating layer 19 and the second insulating layer 20 are selectively etched to open the metal pad 24. A thermal annealing process is then performed. Microlenses 22 are formed on the second insulating layer 20 corresponding to the photodiodes 11, 13 and 15.
The second insulating layer 20 used as the passivation layer may be formed of a nitride layer, and the microlenses 22 may be formed of photoresist. A lifting effect is likely to occur due to poor adhesion between the nitride layer and the photoresist layer. Also, if the metal lines are formed in the path of light, diffused reflection occurs, thereby deteriorating the quality of the image sensor.
The related art CMOS image sensor and the method for fabricating the same have the following problems.
Since the microlenses are primarily formed of photoresist on the nitride layer, a lifting effect may occur due to poor adhesion between the nitride layer and the microlenses. Such a lifting effect of the microlenses causes moving particle to generate poor pixels, thereby reducing yield. Also, since the microlenses are spaced apart from each other, they are easily detached from the nitride layer.
Further, if the metal lines are formed in the path of light, diffused reflection occurs, thereby deteriorating the quality of the image sensor.